ISIS: a new N-body cosmological code with scalar fields based on RAMSES: code presentation and application to the shapes of clusters

Several extensions of the standard cosmological model include scalar fields as new degrees of freedom in the underlying gravitational theory. A particular class of these scalar field theories include screening mechanisms intended to hide the scalar field below observational limits in the solar system, but not on galactic scales, where data still gives the freedom to find possible signatures of their presence. To make predictions to compare with observations coming from galactic and clusters scales (i.e. in the non-linear regime of cosmological evolution), cosmological N-body simulations are needed, for which codes that can solve for the scalar field must be developed. We present a new implementation of scalar-tensor theories of gravity that include screening mechanisms. The code is based on the already existing code RAMSES, to which we have added a non-linear multigrid solver that can treat a large class of scalar tensor theories of modified gravity. We present details of the implementation and the tests that we made to the code. As application of the new code, we studied the influence that two particular modified gravity theories, the symmetron and f(R) gravity, have on the shape of cluster sized dark matter haloes and found consistent results with previous estimations made with a static analysis.